Materials containing silicon (silicon materials) can store lithium ions in a larger amount per unit volume than the currently used carbon materials such as graphite. Because of their high capacity, the use of such materials for negative electrode active materials has been studied. However, the fact that the silicon materials exhibit a large volume change during the insertion and extraction of lithium ions causes a problem that negative electrode active materials including the silicon materials are broken into fine powders by repeated charging and discharging to make it impossible to obtain good cycle characteristics.
In light of such circumstances, for example, it has been proposed that lithium metasilicate (Li2SiO3) or silicon dioxide (SiO2) is used as a negative electrode active material (see Patent Literature 1). It is also proposed that a silicon composite powder in which 2 wt % to 36 wt % silicon fine crystals are dispersed in silicon dioxide is used as a negative electrode active material (see Patent Literature 2).